As is well known, a telescope is an optical device that is designed to form images of far-off objects. A telescope may be constructed from a primary reflector lens that collects and focuses incoming light and one or more secondary lenses that use the collected light to form an image. For such a telescope high spatial resolution, i.e. small spot size is desirable property.
In a telescopic spectroscope, the telescope contains a spectrometer with an input slit located to receive light for spectroscopic analysis from the image plane of the spectroscope, so that spectra can be measured for selectable image plane locations or regions of image plane locations. Spectra may be obtained simultaneously for different locations on a line of locations in the image plane along the length of the opening slit of the spectrometer (in parallel with the lines of the grating, if a grating is used in the spectrometer). In a satellite, or other moving platform, this line is preferably directed transverse to the direction of travel of the platform, so that there is no need for detection at different image plane locations in a direction transverse to the line, because object points imaged at these locations will be imaged on the slit at another time points as a result of movement of the telescopic spectroscope. A large field of view along this line makes it possible to obtain spectra for a wide strip of locations. For such a telescopic spectroscope, high spatial resolution along the line is desirable property.
It may be desirable to minimize the number of lenses in a telescope. For example, it may be desirable to reduce the weight of telescopes for space applications, but it may be desirable to minimize the number of lenses also for other reasons.
U.S. Pat. No. 5,841,575 describes a telescope that requires only two concave reflector lenses. A first reflector lens collects incoming light and forms a virtual image between it and a second reflector lens. The second reflector lens images the virtual image in a final image plane, where a spectrometer receives the resulting light. The first reflector lens reflects the light off-axis towards a second reflector lens: the incoming and outgoing optical axis are at an angle to each other, so that the second reflector lens does not obstruct the incoming rays of the first reflector lens.
An aperture stop is included between the two reflector lenses. The distance between the aperture stop and the second reflector lens corresponds to the focal distance of the second reflector lens. This has the effect that central rays of the light at different positions on the final image plane are parallel, or in other words, that the telescope is telecentric. The document mentions that reflectors with spherical surfaces are preferred, but that aspherically shaped surfaces are also possible.
An additional advantageous effect of the aperture stop, not mentioned in this document, is that it reduces optical aberrations of the telescope such as spherical aberration, coma and, to a limited extent, astigmatism. The aperture stop reduces the range of angles of rays from the first reflector lens that contribute to the final image. Typically, aberrations are proportional to the size of this range and hence the aperture stop reduces the aberrations.
U.S. Pat. No. 5,841,575 describes that the resulting telescope has a large field of vision. An example of ninety degrees is mentioned. However, the spot size at the edge of the field of vision has been found to be quite poor.
It has been known to realize telescopes that have different magnification along different directions in the image plane. Such a telescope is called an anamorphotic telescope. Typically an image plane direction of maximum magnification and an image plane direction of minimum magnification can be defined in an anamorphotic telescope. By tracing back along the optical path through the telescope, corresponding directions of maximum and minimum magnification can be identified everywhere along the optical path. A vector between a pair of image points is magnified according to the different magnifications of its components along the directions of maximum and minimum magnification, at least in the limit of image points near the optical axis (called the paraxial limit).
An anamorphotic telescope may be realized for example by using lenses that each have mutually different focal distances for object lines transverse to the different magnification directions respectively. These lenses can be placed at such a distance that a first lens images the object lines at different intermediate virtual image surfaces between the lenses, and that a second lens images the object lines from the different virtual image surfaces to the same final image plane. This has the effect that the magnification of the object lines in the different directions onto the final image plane will be different.
A lens that has mutually different focal distances for object lines in the different directions can be realized by means of a lens surface that has mutually different radii of curvature in the directions of minimum and maximum magnification, as traced back along the optical path. Each lens may have a basic toroidal shape, optionally with small deviations from a perfect toroidal shape.